The present invention relates to a method for signaling a decrease in tire air-pressure and an apparatus used therefor. More particularly, the present invention relates to a method for signaling a decrease in tire air-pressure and to an apparatus used therefor capable of properly judging that decrease in internal pressure has occurred in any of tires attached to a vehicle equipped with a limited slip differential device (LSD) and accordingly generating an alarm.
It is conventionally known that a dynamic load radius of a tire becomes small in the case where the air-pressure in the tire is decreased, and that the rotational velocity or rotational angular velocity becomes faster than that of a tire having normal air-pressure. For instance, there is proposed a method for detecting a decrease in the internal pressure of a tire based on the relative differences in the rotational angular velocities of the tires (see, for instance, Japanese Unexamined Pat. Publication No. 305011/1988).
A method for judging an alarm in a conventional apparatus for detecting a decrease in air-pressure is generally performed through the following steps.
1) A judgment value is calculated at each specified interval (e.g., every second). PA1 2) In case a specified amount of judgment values have been stored (e.g., five values), an average value of these values is calculated. PA1 3) In case a specified amount of average values have been stored (e.g., twelve values), another average value of these values is calculated and decompression is judged on the basis of this average value.
In the above arrangement, each of the values which have been obtained in the steps of 1) to 3) are stored in RAMs (isokinetic random access memory devices), while judgment values obtained in the decompressed condition, for instance, twelve values remain in some of the RAMs when the process returns from a step of judging decompression of a tire to a step of judging normal internal pressure. Therefore, there exists a fear that it might be judged that a judgment value close to a decompressed condition is a false alarm due to variations in judgment values for running data obtained after judging that a normal internal pressure has been assumed even though the tire might be in a normal condition.
Among recent vehicles, there are some which are equipped with a limited slip differential device at a differential gear of a driving shaft for the purpose of improving running performances at the time of, for instance, cornering. The mechanism of the limited slip differential device is so arranged that the differential is restricted until a differential torque exceeds a set value and the driving wheels are rotated at identical velocities on the right and left.
In a vehicle equipped with such a limited slip differential device, judgment of alarm in the above steps 1) to 3) is performed separately for a region in which differential is limited (straight line region) and for a region in which differential is not limited (turning region). Although methods for judging an alarm are somewhat different from each other, there also exists the danger that a value is judged to be a false alarm due to similar reasons as for conventional apparatus for detecting a decrease in air-pressure.
The present invention has been made in view of these facts, and it is an object of the invention to provide a method for determining a decrease in tire air-pressure and an apparatus used therefore capable of properly determining the decrease in internal pressure of a tire.